...removed from the front right tyre of my car. Which makes me think, never had nails sticking into the tread but often pick up screws. Why should this be ?

Interesting factlet: puncture repairs seem to be getting faster and faster. From wheel off car to wheel back on took fifteen minutes (i.e. screw removed, tyre deflated, tyre removed, drilled, plugged, tyre back on wheel) which I found most impressive.